Most frequent travellers suffer from jet lag (dysrhythmia), which occurs when the body clock is not synchronised with an air traveller's new time zone, causing the disruption of more than 50 physiological and psychological rhythms. Studies show that jet lag worsens with age, particularly after age 50. Until recently, jet lag was dismissed as merely an unpleasant side-effect of air travel, but new research suggests that it also causes memory loss, shrinkage of parts of the brain and negative side effects on blood pressure. In one study, jet lag has even been implicated in the incidence of cancer. The common symptoms of jet lag are fatigue, poor concentration, trouble sleeping, irritability, minor depression, altered estimation of time and distance and digestive problems.